1. Field of the Invention
The present invention relates to an image forming apparatus including a light scanning device which directs light beams emitted from a plurality of laser light sources onto photoconductive members via an image focusing device having optical elements and a deflection device which deflects the light in a main scanning direction; an image forming device which forms visible images of different colors from the images written onto a plurality of photoconductive members by the light scanning device; and an intermediate transfer member or recording sheet transport member that can support the visible images formed on the photoconductive members.
2. Description of the Background Art
In a tandem type image forming apparatus that forms images of each color at the same time using a single polygon motor, registration adjustment is carried out between colors to prevent reduction in image quality due to color registration errors. The timing of this registration adjustment is at the time of writing, so it is only possible to adjust at scan time intervals of one face of the polygon mirror, so there is a possibility of occurrence of a maximum of one line of color registration error.
Also, in recent years it has been necessary to reduce the cost of the light scanning device, so plastic has been adopted instead of die casting, so the use of resin forming has become essential. In particular, the number of components in the optical elements of the tandem type writing unit is large, so the cost reduction effect of changing to plastic is very large, so plastic has been adopted for forming the optical elements. However, bending can easily occur in the long direction of long plastic optical elements, in particular in the direction normal to the main scanning direction. This bending is several tens of microns, and due to differences in the molds the magnitude and direction of the bending varies. Hence it was very difficult to align between each of the stations with high accuracy scan lines with bending and inclination. For example, if the bending in one color is 30 μm and this color is set to be the standard color, even if the other colors are aligned to the bending a maximum of 60 μm registration error occurs. Also, when an actual image is output, curvature is produced at the transfer unit, photoconductive member, and so on, so even if color registration errors are eliminated from the writing unit, color registration errors can still occur. This type of color registration error can occur while operating, and cause deterioration of the image quality.
Therefore, in for example Japanese Patent Application Laid-open No. 2004-287380 the beam position in the sub scanning direction is detected, and inclination or bending of the image is dealt with by inclining or bending the scan lens. However, correction of color registration errors after carrying out the color registration error detection operation is not clearly described.
Furthermore, in this Japanese Patent Application Laid-open No. 2004-287380 technology is disclosed for correcting scan line bending by scan line bending correction means provided in the retaining member that supports the optical elements. However, in this conventional art one of the four colors is the standard, and the scan position of the scan beams of the colors apart from the standard are corrected to substantially coincide with the scan position of the standard color. Therefore if the scan line bending of the standard color increased due to thermal deformation, the correction deviation to the standard color may become too large.
Also, in Japanese Patent Application Laid-open No. 2004-174720, a light exposure device is disclosed that includes a light source and an optical scanning system that deflects and reflects the light beam emitted from the light source towards a light exposure member as well as scanning the light exposure member. Specifically, the light exposure device includes a deflecting member disposed in the light path formed by the light scanning system, and control means connected to the deflecting member that control the deflection of the light beam deflected by the deflecting member. The deflecting member deflects the light beam emitted from the light scanning system towards the light exposure member. The control means measures the amount of positional deviation between electrostatic latent images when electrostatic latent images are formed on different light exposure members, and controls the deflection of the light beam by the deflecting member based on the amount of positional deviation. However, this device cannot respond to the deviation of laser beam irradiation position caused by the heat generated by a motor such as a polygon motor.
Also, in Japanese Patent Application Laid-open No. 2003-94719, a mechanism that carries out registration adjustment to one line or less in the sub scanning direction by rotating an LD light source is disclosed. However, this does not deal with correcting inclination or bending.
Furthermore, in Japanese Patent Application Laid-open No. 2001-253113 and Japanese Patent No. 3087748, technology is disclosed for detecting the beam position in the sub scanning direction. However, this conventional art does not deal with inclination and bending with one line registration adjustment.
In this way, registration between colors, scan line inclination between colors, and scan line bending between colors due to variations in the scan position of the laser beam can cause color registration errors which cause image deterioration. It has been difficult for conventional art to correct these errors with high accuracy.